Live load reduction history 3

[trevorshiloh]

I deal with a lot of historic structures and part of trying to reproduce the design without existing drawings is figuring out whether live load reduction was used.

A quick search didn't yield any meaningful results, and the ASCE 7 commentary mentions influence area being introduced in 1982 with the concept of live load reduction introduced "over 40 years ago", which I'm assuming is 40 years ago from 1982 since 7-05, 7-10, and 7-16 all cite the same number.

Wondering if anyone knows of any references explaining when it was first conceptualized/codified, how it has evolved, and what they did before influence area.

 

[lexpatrie]

This is a very broad question -

You really can't diagnose this live load reduction off a plan, typically, as there's no requirement (not even currently) for disclosing if live load reduction was used, and for older buildings the general notes tend to not exist anyway, and based on what I've seen, it's not voluntarily disclosed.

You can determine it, at least potentially to your satisfaction, via recreating the calculations based on the design information you can find. It will be complicated by the quality of the original design work, however, as some of this stuff wasn't all that designed as it was "look and feel" if you go far enough back (and even today we have some engineers who "design" things without calculations, i.e. the Haug disciplinary action in Florida).

I would presume live load reduction is used on the typical structure. It might have been used incorrectly, or overapplied, of course.

The older codes (BOCA, SBC, UBC) had what are likely different provisions for live load reduction, and they were different from the modern ones if you go back far enough it was just a blanket reduction based on the number of supported floors in the SBC, for example.

1984 South Florida Building Code, with Broward amendments

Then you have potential changes in the load factors, if you have old enough concrete structures, those have changed several times, as well as the load combinations, and the old 33% stress increase can also figure into things, if you have wind loads, for instance.

I've not done a deep dive on the subject, and I'm not clear in the ICC process which code "won" on live load reduction, was it UBC, BOCA, SBC, or something else). But you can do this for a particular project (and really should) so you can evaluate if it was done reasonably well before you start fiddling with the structure any further.

If you like really odd graphics and lots of lines..... here's something:
A Systematic Approach to Live Load Reduction for Columns, Casey, P.E., Structure Magazine, June 2023.

It does not go into any background, or the references section doesn't disclose it.

 

[SlideRuleEra]

Wondering if anyone knows of any references explaining when it was first conceptualized/codified, how it has evolved, and what they did before influence area.

I am always interested in the mix of history & engineering (you might have guessed that). So, looked to see what is available:

First, looking at the 1945 American Standard Building Code:

A version of live load reduction was used:

In an older document there is the careful documentation of investigations preformed for the U.S. Department of Commerce to determine logical requirements for live loads. Summary of results provided in 1924 Minimum Live Loads Allowable for Use In Design of Buildings Live load reduction is just reduction using the number of floors, as described by lexpatrie, but otherwise this publication appears to be the fundamental basis for many, if not all modern building codes:

Here is an excerpt from this document's introduction:

 

[lexpatrie]

Also, at least Chicago and New York which (?) had building codes prior to 1926, and may have had provisions for live load reduction there, although it appears live loads were "rationalized" partly due to those differences between the various codes in the larger cities.

I know I've seen tables showing differing live loads based on "ancient" (I suppose I mean colonial and pre-colonial) cities here in the U.S. (Any body know where I saw that table??)

I feel sometimes like slide rule and I share a reference library.

 

[jerseyshore]

Here's the 1916 NYC code excerpt:

 

[lexpatrie]

Slide rule - the links in your post, well, the 1924 one doesn't work (it looks like it's to your Google Drive, by the way), and the other one to the 1945 is information on the book from google books. Both of these documents are "out there" free to download. If you wanted to change the links, I've got them both in the FAQs for this part of the forum, as of today.

 

[trevorshiloh]

Wow, thanks everyone for the great reads. Always interesting to see what kind of language was used to try and describe a complex concept in the clearest way possible (and sometimes failing to, or overcomplicating it). Will definitely keep these as references for future projects.

Also, I need to stop forgetting this place exists right after posting. I've done that twice now.

 

[jerseyshore]

I've said this many times, but whenever I need to reference the older NYC codes I'm jealous of the simpler times designers and engineers had when it comes to code language. The entire load section is a few pages. Used to say wind load = 25 psf (or something like that). Boom whole city 25 psf. Now we have like 58 ASCE 7 wind chapters and your head can spin trying to find the right section.

 

[lexpatrie]

Well I didn't find the document, but I did find a table posted by JAE that showed live loads for various cities "back when".

Find it here:

https://www.eng-tips.com/viewthread.cfm?qid=516865

 

[EngDM]

You really can't diagnose this live load reduction off a plan, typically, as there's no requirement (not even currently) for disclosing if live load reduction was used, and for older buildings the general notes tend to not exist anyway, and based on what I've seen, it's not voluntarily disclosed.

For the Canadians out there, the NBCC does require disclosure of if live load reduction factors were used. NBCC 4.1.5.8 sentence 5).

 

[trevorshiloh]

For the Canadians out there, the NBCC does require disclosure of if live load reduction factors were used. NBCC 4.1.5.8 sentence 5).

Sure would be nice if we did that over here.

I often find myself criticizing things in existing drawings just to realize the same things apply to mine.

 

[lexpatrie]

I managed to get my scanner to limp back to life and guess the correct buttons to push in the dim light.

Anyway.

Not exactly on the topic, but adjacent at least, here's a live load table from Design in Structural Steel, Lothers, 1953 (archive.org).

 

[lexpatrie]

Sorry guys I can't edit my previous post, the reference is wrong. This is from Structural Steel Design, Lambert Tall, chief editor, Lynn Beedle, and Ted Galambos, Fritz Engineering Laboratory, Ronald Press Company, New York, 1964. Page 50. I scanned it downstairs then decided what book it came from upstairs, because I kind of wasn't expecting to correctly guess the scanner buttons correctly in the dark.

In my glee to upload the images, I got the references wrong on two posts in the forum.

Again, sorry.